Global Warming History Is Written In Trees

Tree samples from Eastern Europe confirm that the recent warming in the region is outpacing any natural climate variations seen since the Middle Ages -- and also reveal that some of the biggest plague outbreaks and political conflicts occurred during colder periods.

Establishing the history of climate that occurred before we had satellites and weather stations at our disposal is difficult. But scientists can often ascertain historical variations in temperatures from indirect kinds of evidence. Climate leaves its traces in rocks, ice sheets and even trees.

The science of deducing climate from trees, or “dendroclimatology,” involves studying the annual rings that can be seen in horizontal cross-sections of the trunk. Wider tree rings generally correspond with better growing conditions, while thinner rings mean leaner times.

While there are some European climate records that reach back to medieval times, there isn’t much hard evidence for climate conditions in Eastern Europe.

To fill in the gap, researcher Ulf Buntgen and his team took samples from 282 living larch trees and 283 pieces of larch timber used in construction, all from a northern Slovakian forest region. Some of the samples dated all the way back to 963 A.D. Examining the rings of the tree samples, the scientists were able to get a picture of how temperatures from May to June varied from year to year and century to century in that region.

Their results, published on Monday in the Proceedings of the National Academy of Sciences, show that recent decades have been very different than the rest of the 972-year-long period studied. Twenty of the 33 warmest years occurred after 1960, and the warmest May-June period recorded was in 2011. The warmest 30-year interval was from 1957 to 1989, according to the paper.

Or, to put it more succinctly, recent warming attributed to human activity “exceeds the range of past natural climate variability,” the authors wrote.

Eastern Europe in the Middle Ages was a much chillier place, it turns out. The coldest period the researchers found was in the spring of 1248. Buntgen and his colleagues also found an interesting correlation between prolonged cold periods and past social and political upheaval.

“The Black Death in the mid-14th century, the Thirty Years War in the early 17th century and the French invasion of Russia in the early 19th century all occurred during the coldest episodes of the last millennium,” the authors wrote.

SOURCE: Buntgen et al. “Filling the Eastern European gap in millennium-long temperature reconstructions.” Proceedings of the National Academy of Sciences published online 14 January 2013.